Calibration device and calibration method for detector

ABSTRACT

Provided is a calibration method for a detector including a plurality of detecting elements each of which generates an electrical pulse signal with a peak value corresponding to an energy value of incident X-ray photons and counts a photon count for each peak value, including: applying a predetermined tube voltage to an X-ray tube for irradiating the detector with X-rays; acquiring the photon count for each peak value from each of the detecting elements; estimating a maximum peak value H within a range in which X-ray photons are detected and at which the peak value is maximum; and calculating, for each of the detecting elements, a calibration value that associates the tube voltage with the maximum peak value H.

TECHNICAL FIELD

The present invention relates to a calibration device and calibrationmethod for a detector having multiple detecting elements or detectionpixels (hereinafter collectively referred to as detecting elements) thatgenerate electrical pulse signals with peak values corresponding to theenergy values of X-ray photons and count the photon count for each peakvalue. More specifically, the present invention relates to a calibrationdevice and calibration method that can accurately perform calibration ofthe energy values measured by the detecting elements.

BACKGROUND ART

Various X-ray devices have been proposed (see Patent Document 1 forexample). Patent Document 1 discloses a configuration of a detector thatincludes multiple detecting elements and can acquire the photon countand energy values of X-ray photons for each detecting element. Thedetector is configured to generate an electrical pulse signal from thedetecting element when X-ray photons are incident, and convert the peakvalue of this electrical pulse signal into the energy value of thephotons.

In addition, the X-ray device described in Patent Document 1 has anenergy discrimination function. Multiple energy values are set asboundaries, and multiple ranges of energy values (energy bins) ispredetermined. The detector has the function to discriminate whichenergy bin the incident photons belong to for each detecting element. Byusing different correction data for each energy bin and performingcorrections, it is possible to improve the measurement accuracy of theX-ray device.

In order to determine which energy bin the energy values of photonsdetected by the detecting elements belong to, each detecting element isrequired to accurately measure the energy values of photons,particularly at the boundary energy values of the energy bins. In otherwords, when multiple photons with a specific energy value enter multipledetecting elements, one photon per element, the same energy value shouldbe obtained from each detecting element. Here, whether simultaneous ordiscrete in time, the same energy value should be obtained. In detectorsperforming energy discrimination with set energy bins, it is importantthat the boundaries of the energy bins belonging to each detectingelement align between the detecting elements, and ideally, match thedesired energy intended by the operator of the detector.

However, due to the influence of accuracy and other factors during themanufacturing of detecting elements, there is actually a variation inthe energy values obtained for each detecting element. By calibratingthe relationship between the peak values and the energy values outputtedby each detecting element and obtaining accurate energy values, it ispossible to further improve the measurement accuracy of the X-raydevice.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Patent No. 6590381

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

The present invention has been made in view of the above problems, andan object thereof is to provide a calibration device and a calibrationmethod that can accurately perform calibration of the energy valuesmeasured by the detecting elements.

Means for Solving the Problem

A calibration device to achieve the object is a calibration device for adetector including a plurality of detecting elements each of whichgenerates an electrical pulse signal with a peak value corresponding toan energy value of incident X-ray photons and counts a photon count foreach peak value, the calibration device including: an X-ray tube controlunit that controls a tube voltage of an X-ray tube for irradiating thedetector with X-rays; an acquisition unit that acquires the photon countfor each peak value from each of the detecting elements and acquires thetube voltage of the X-ray tube; a calculation unit that estimates amaximum peak value within a range in which X-ray photons are detectedfrom values obtained by the acquisition unit and at which the peak valueis maximum; and a calibration unit that calculates, for each of thedetecting elements, a calibration value that associates the tube voltageacquired by the acquisition unit with the maximum peak value.

A calibration method to achieve the object is a calibration method for adetector including a plurality of detecting elements each of whichgenerates an electrical pulse signal with a peak value corresponding toan energy value of incident X-ray photons and counts a photon count foreach peak value, the calibration method including: applying apredetermined tube voltage to an X-ray tube for irradiating the detectorwith X-rays; acquiring the photon count for each peak value from each ofthe detecting elements; estimating a maximum peak value within a rangein which X-ray photons are detected and at which the peak value ismaximum; and calculating, for each of the detecting elements, acalibration value that associates the tube voltage with the maximum peakvalue.

Effects of the Invention

According to the present invention, it is possible to calibrate thedetecting elements using the maximum peak value. The maximum peak valuecorresponds to the maximum value of energy that X-ray photons can obtainfrom the X-ray tube, and corresponds to the tube voltage. This isadvantageous for accurately calibrating the energy values measured bythe detecting elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory diagram illustrating an overview of acalibration device connected to an X-ray device.

FIG. 2 is an explanatory diagram illustrating the state of peak valuesoutputted from a detecting element.

FIG. 3 is a graph illustrating the relationship between the photon countand the peak value.

FIG. 4 is a graph illustrating an enlarged section surrounded by adashed circle in FIG. 3 .

FIG. 5 is a graph illustrating the relationship between the square rootof photon count and the peak value.

FIG. 6 is a graph illustrating the relationship between the square rootof photon count and the peak value when measured multiple times withchanged tube voltages.

FIG. 7 is an explanatory diagram illustrating the relationship betweenthe peak value and the energy value in a detecting element.

FIG. 8 is an explanatory diagram illustrating the configuration of adetector.

FIG. 9 is a graph illustrating the relationship between the outputtedchannel number and the number of detecting elements.

FIG. 10 is an explanatory diagram illustrating a modification of thecalibration device of FIG. 1 .

FIG. 11 is a graph illustrating the relationship between the square rootof photon count and the peak value when a filter mechanism is used.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, a calibration device and a calibration method for adetector will be described based on the embodiments shown in thedrawings.

As illustrated in FIG. 1 , a calibration device 1 is used by beingconnected to an X-ray device 2. The X-ray device 2 is used in foodinspection and medical practice. The X-ray device 2 includes an X-raytube 3 that emits X-rays, a detector 4 that detects the X-rays emittedfrom the X-ray tube 3, and a control mechanism 5 that controls the X-raytube 3 and the detector 4.

The X-ray tube 3 is configured to emit continuous X-rays by beingsupplied with electricity having a predetermined voltage value andcurrent value under the control of the control mechanism 5. The X-raytube 3 emits X-rays toward a measurement target object arranged betweenthe X-ray tube 3 and the detector 4.

The detector 4 is configured to detect X-rays emitted from the X-raytube 3 and transmitted through the measurement target object. Thedetector 4 includes a plurality of detecting elements 6 arranged side byside in a plane. The detecting element 6 is configured to generate anelectrical pulse signal with a peak value corresponding to the energyvalue of incident X-ray photons and to count the photon count. Thedetecting element 6 is composed of a direct conversion semiconductor,such as a CdTe (cadmium telluride) based semiconductor, and asemiconductor such as a photon counting type ASIC (application specificintegrated circuit) that amplifies and digitizes signals from the directconversion semiconductor.

The configuration of the detecting element 6 is not limited to theabove. The detecting element 6 may have a configuration that can acquirethe energy value and photon count of X-ray photons.

The control mechanism. 5 is, for example, a computer. The controlmechanism 5 is configured to control the voltage value and current valueof electricity supplied to the X-ray tube 3. The control mechanism 5 isconfigured to acquire data from the detector 4. This data is used todisplay an X-ray image on a monitor or the like connected to the outsideof the X-ray device 2, for example. The control mechanism 5 is connectedto the X-ray tube 3 and the detector 4 by signal lines. In FIG. 1 , thesignal lines are indicated by dashed lines for explanation. In addition,the arrow indicates the direction of emitting X-rays.

The calibration device 1 is connected to the control mechanism 5 of theX-ray device 2 via a signal line. The calibration device 1 may beconfigured to be incorporated inside the X-ray device 2.

The calibration device 1 includes an X-ray tube control unit 7 thatcontrols the tube voltage of the X-ray tube 3. The X-ray tube controlunit 7 can act on the control mechanism 5 to control the electricvoltage (tube voltage) supplied to the X-ray tube 3. The X-ray tubecontrol unit 7 may be configured to control the electric current (tubecurrent) supplied to the X-ray tube 3 in addition to the tube voltage.

The calibration device 1 includes an acquisition unit 8 that acquiresthe photon count for each peak value from a plurality of detectingelements 6 forming the detector 4. In the present embodiment, theacquisition unit 8 acquires the photon count for each peak valueacquired by the control mechanism 5 from the detector 4. The acquisitionunit 8 is also configured to acquire the tube voltage from the X-raytube control unit 7. The acquisition unit 8 may be configured to acquirethe tube voltage from the control mechanism 5.

The calibration device 1 includes a calculation unit 9 that estimates amaximum peak value H within a range in which X-ray photons are detectedfrom values obtained by the acquisition unit 8 and at which the peakvalue is maximum. The maximum peak value His estimated by thecalculation unit 9 for each detecting element 6.

The calibration device 1 includes a calibration unit 10 that calculatesa calibration value d that associates the tube voltage obtained by theacquisition unit 8 with the maximum peak value H estimated by thecalculation unit 9. The calibration value d is calculated by thecalibration unit 10 for each detecting element 6. The calibration valued obtained by the calibration unit 10 is sent to the control mechanism 5and stored therein. The control mechanism 5 can calibrate the dataacquired by the detector 4 with the calibration value d. Additionally,when the detector 4 has a fine adjustment mechanism for peak value andpeak value detection threshold, the control for fine adjustment can beperformed based on the calibration value d, allowing the energy valueoutput itself to be adjusted to the same value.

The X-ray tube control unit 7 and the like included in the calibrationdevice 1 are connected by signal lines. In FIG. 1 , the signal line isindicated by a dashed-dotted line for explanation. Signal lines used forthe calibration device 1 and the X-ray device 2 may be wired orwireless.

Next, a method for calibrating the detector 4 will be described. First,the calibration device 1 is connected to the X-ray device 2 with asignal line or the like. The X-ray tube control unit 7 determines theelectric voltage to be supplied to the X-ray tube 3 and the like. Forexample, the X-ray tube control unit 7 supplies electricity to the X-raytube 3 through the control mechanism 5 so that the tube voltage is 40 kVand the tube current is 0.5 mA. For example, by arranging an aluminumfilter between the X-ray tube 3 and the detecting element 6, or byadjusting the distance between the X-ray tube 3 and the detectingelement 6, a low dose of X-rays, for example, 300 CPS (counts persecond), is emitted from the X-ray tube 3 to the detector 4 onto thedetecting element 6. In the embodiment shown in FIG. 1 , nothing isarranged between the X-ray tube 3 and the detector 4. Measurement by theX-ray device 2 is continued for a predetermined measurement time, suchas two hours.

As illustrated in FIG. 2 , each time X-ray photons are incident, thedetecting element 6 of the detector 4 generates an electrical pulsesignal with a peak value corresponding to the energy value of thephotons. The larger the energy value possessed by X-ray photons, thelarger the peak value of the electrical pulse signal sent from thedetecting element 6 to the control mechanism 5. Note that theaforementioned low dose of X-rays is a dose at which the pile-upphenomenon can be ignored during photon counting. There is no generaldefinition of what CPS is low dose. If the dose is 300 CPS, the intervalbetween X-ray photons incident on each detecting element 6 is 3.3 ms onaverage. In this case, the pulse interval p of electrical pulse signalsis 3.3 ms on average. For example, when the pulse width w is 300 ns,average (p/w)=10000, and the pulse width w becomes significantly shortercompared to the pulse interval p, making the pile-up phenomenon almostnegligible. As above, in the present specification, a state where thepile-up phenomenon can be almost ignored is referred to as an ultra-lowdose state. Also, in the present specification, a state with an average(p/w) of 100 or more is referred to as a low dose state.

The acquisition unit 8 of the calibration device 1 acquires the photoncount for each peak value via the control mechanism 5. The acquisitionunit 8 can acquire a graph illustrating the relationship between thephoton count and the peak value, as illustrated in FIG. 3 . The photoncount for each peak value is, in practice, the photon count with a peakvalue equal to or greater than a certain peak value hx, and in thepresent specification, the photon count with a peak value that fallswithin the range of the infinitely high peak value (h=∞), which existsonly by definition, and the peak value hx is denoted as C(∞:hx). Asanother example, the photon count representing a certain range of peakvalues, the photon count with a peak value that falls within the rangeof peak value hx1 and peak value hx2, is denoted in a similar notationas C(hx1:hx2).

Next, based on the data acquired by the acquisition unit 8, thecalculation unit 9 estimates the maximum peak value H. As illustrated inFIG. 3 , when the vertical axis is the photon count and the horizontalaxis is the peak value, the peak value when the photon count is zero isdefined as the maximum peak value H in the present specification. Themaximum peak value H is the value of the peak value within a range inwhich X-ray photons are detected and at which the peak value is maximum.

In the present embodiment, the photon counting type ASIC included in thedetecting element 6 is configured to acquire an integrated valueobtained by integrating the photon count up to infinity height withrespect to the peak value (photon energy). It is preferable to use aphoton counting ASIC that acquires integral values such as C (∞:hx)rather than difference values such as C (hx1:hx2), as it results insmaller measurement errors and improves the calibration accuracy. Thisis because the difference calculation or the use of the differencecalculation circuit inside the ASIC increases errors due to the errorpropagation law during the difference calculation. A photon countingtype ASIC included in the detecting element 6 may be configured toacquire difference values. Note that if the photon counting type ASIC isconfigured to acquire integral values, the calculation unit 9 may beconfigured to acquire difference values by calculation.

The range in which photons are detected refers to the range of peakvalues in which X-ray photons can be detected. It can be said that therange in which photons are detected is a range excluding a range inwhich the photon count is obviously zero from the shape of the graphillustrated in FIG. 3 , for example. The range in which photons aredetected is not limited to the range in which the photon count is 1 ormore. Even if the photon count actually detected is zero, it may beincluded in this range if there is a possibility that photons could bedetected.

Theoretically, the maximum peak value H does not exceed the energy valueof the tube voltage of the X-ray tube 3 in the case of an ultra-low dosestate, in which the pile-up phenomenon is negligible. This is becausethe X-ray tube 3 cannot generate X-ray photons having energy higher thanthe energy obtained from the tube voltage. For example, when the tubevoltage is 40 kV, the maximum energy value of X-ray photons is 40 keV.Note that when a pile-up phenomenon occurs, two pulses are synthesized,so that the peak value of the pulse may exceed the maximum peak value H.

As illustrated in FIG. 3 , the maximum peak value H detected by acertain detecting element 6 corresponds to the tube voltage. Therefore,when the tube voltage is 40 kV, the maximum peak value H corresponds toan energy value of 40 keV. During the measurement of the target objectby the X-ray device 2, when this detecting element 6 detects anelectrical pulse signal that matches the maximum peak value H, it can besaid that the energy value of the X-ray photons detected by thisdetecting element 6 is 40 keV.

The calibration unit 10 calculates a calibration value d that associatesthe tube voltage acquired by the acquisition unit 8 with the maximumpeak value H obtained from the calculation unit 9. This calibrationvalue d is sent to the control mechanism 5 of the X-ray device 2 andstored therein. When measuring the target object, the X-ray device 2 canperform calibration using the calibration value d for the data obtainedfrom the detector 4, and can then utilize this calibration whenoutputting measurement results such as X-ray images.

Calibration by the calibration device 1 enables all the detectingelements 6 to output accurate energy values at least for the energyvalue corresponding to the maximum peak value H. The energy values of aplurality of detecting elements 6 can each be calibrated with respect toone index, tube voltage. When X-ray photons having the same energy valueare incident, any detecting element 6 will detect the same energy value.The ability to calibrate the energy value obtained from each detectingelement 6 allows energy analysis by the detector 4. It becomes possibleto use the detector 4 as an energy analyzer.

By changing the tube voltage with the X-ray tube control unit 7, itbecomes possible to perform the same measurements as previouslymentioned at different tube voltages. The calibration device 1 changesthe tube voltage, for example, from 40 kV to 80 kV, estimates themaximum peak value H for each detecting element 6, and calculates thecalibration value d. As a result, all detecting elements 6 canaccurately output the energy values of the photons for the X-ray photonswith energy values of 40 keV and 80 keV.

A specific example of estimating the maximum peak value H will bedescribed below. FIG. 4 illustrates an enlarged area surrounded by adashed circle in FIG. 3 . As illustrated in FIG. 4 , in the region nearthe maximum peak value H (extraction region S), it is possible toestimate the maximum peak value H by determining an approximate straightline or approximate curve based on the relationship between the photoncount and the peak value.

The extraction region S is set within a predetermined range that issmaller than the peak value corresponding to the tube voltage. Forexample, when the tube voltage is 40 kV, the extraction region S can beset in the range of 35 keV to 38 keV. In reality, the extraction regionS is a region sandwiched between the peak value corresponding to anenergy value of approximately 35 keV and the peak value corresponding toan energy value of approximately 38 keV. The relationship between theseenergy values and peak values is the value before being calibrated bythe calibration device 1.

As illustrated in FIG. 4 , in the graph illustrating the relationshipbetween the photon count y and the peak value h, the data within therange of the extraction region S is linearly approximated. At this time,the formula for the straight line, y=−ah+b=−a(h−H), is obtained, whereb=a×H. For the photon count to be zero (y=0) in this formula for thestraight line, h=H must be established. The peak value h is determinedas the maximum peak value H. This maximum peak value H corresponds tothe energy value corresponding to the tube voltage.

If the maximum peak value H estimated by the calculation unit 9 is, forexample, a magnitude corresponding to an energy value of 39.8 keV, thecalibration value d for setting this to an energy value of 40.0 keVcorresponding to the tube voltage is calculated by the calibration unit10. For example, let the calibration value d=40.0/39.8. For the peakvalue h obtained from the detecting element 6, the energy value iscalibrated assuming that the energy value corresponding to the peakvalue dh is the true value.

For example, it may be that the calibration value d=40.0−39.8=0.02. Forthe peak value h obtained from the detecting element 6, the energy valueis calibrated assuming that the energy value corresponding to the peakvalue h+d is the true value. The calibration value d is calculated foreach detecting element 6 and calibrated for each detecting element 6with the corresponding calibration value d.

The accuracy of the calibration value d can be improved by setting theextraction region S to a range smaller than the peak value correspondingto the tube voltage. Since the photon count detected by the detectingelement 6 decreases near the maximum peak value H, the measurement errorincreases. The accuracy of calibration can be improved by excluding therange in which the error tends to increase from the extraction region S.For example, the extraction region S may be set by determining the lowerlimit value of the photon count. The configuration may be such that datain which the photon count is 10 or less near the maximum peak value H isexcluded from the extraction region S. This is advantageous forimproving the accuracy of calibration by the calibration device 1.

The reason why the error increases as the photon count decreases is thatthe generation probability of X-ray photons emitted from the X-ray tube3 follows Poisson statistics. The standard deviation of the errordistribution for the photon count C measured in a certain period of timeis √c. The ratio of this standard deviation to the photon count is√c/c=1/√c, so that the more the photon count C decreases, the larger theratio of error.

When determining the lower limit value of the photon count C and settingthe extraction region S, one can first tentatively determine theextraction region S, then calculate the degree of approximation of theapproximate straight line, and perform similar calculations whilechanging the extraction region S, adopting the extraction region S withthe highest degree of approximation. With this method, even if the sizeof the detecting element 6 changes due to intentional or manufacturingvariations or other circumstances, the energy value can be accuratelycalibrated.

When the size of the detecting element 6 (the area of the surfaceirradiated with X-rays) changes, the photon count C increases ordecreases by the area ratio. Therefore, fixing the lower limit value ofthe photon count C is not appropriate in some cases. However, there isno doubt that the ratio of this standard deviation to the photon countC, √c/c=1/√c, affects the error. On condition that near the maximum peakvalue H, data in which the photon count C is less than or equal to afixed value is excluded from the extraction region S, the extractionregion S with the highest degree of approximation can be searched andadopted.

It is desirable to perform calibration by the calibration device 1within a range in which the dose of X-rays emitted from the X-ray tube 3is in an ultra-low dose state, in which no pile-up occurs. Thecalibration by the calibration device 1 may be performed in a rangewhere the X-ray dose is in a low dose state. By setting the tube currentsupplied to the X-ray tube 3 to a relatively small value by the X-raytube control unit 7, an ultra-low dose state or a low dose state can beachieved. The problem that two or more electrical pulse signals aresuperimposed due to pile-up and the peak value appears to be large canbe avoided by using the ultra-low dose state or the low dose state. Itis possible to avoid obtaining a peak value corresponding to an energyvalue exceeding the tube voltage due to pile-up. Specifically, the tubecurrent is set so that the dose is 1000 CPS or less. Desirably, the tubecurrent is set so that the dose is 300 CPS or less. This is advantageousfor further reducing the possibility of pile-up occurring. Bycontrolling the tube current of the X-ray tube 3 with the X-ray tubecontrol unit 7, the dose can be easily adjusted.

As illustrated in FIG. 5 , the configuration may be such that theacquisition unit 8 acquires a graph illustrating the relationshipbetween the square root of photon count C(∞:h) and the peak value h. Thegraph illustrating the relationship between the square root of photoncount C(∞:h) and the peak value h is closer to a straight line than thegraph illustrated in FIG. 3 both theoretically and practically.Therefore, it is possible to improve the accuracy when determining theapproximate straight line based on the relationship between the squareroot of photon count and the peak value. Errors in estimating themaximum peak value H can be further suppressed.

The method of estimating the maximum peak value H is not limited to theabove. The configuration may be such that the approximation is madeusing a curve, rather than a straight line, in the extraction region S(for example, polynomial approximation). Alternatively, for example, thegraph illustrating the relationship between the square root of photoncount C and the peak value h illustrated in FIG. 5 may be generated, andimage processing may be used to estimate the maximum peak value H. Ifthe maximum peak value H can be obtained with relatively high accuracy,the maximum peak value H may be estimated by a method other than theabove.

A calibration method when the detecting element 6 has an energydiscrimination function will be described below. For example, it isassumed that five energy value ranges (energy bins) are set in advance,and the energy values at the boundaries of the energy bins are set to 20keV, 40 keV, 60 keV, and 80 keV.

As illustrated in FIG. 6 , the detector 4 first detects X-ray photonsfor a predetermined time, such as two hours, while the tube voltage iscontrolled at 20 kV by the X-ray tube control unit 7. Using the dataobtained by the detector 4 and the like, the calculation unit 9estimates the maximum peak value H1. A maximum peak value H1 isestimated for each of the plurality of detecting elements 6 constitutingthe detector 4. A calibration value d is calculated for each detectingelement 6 in the calibration unit 10.

Next, the X-ray tube control unit 7 changes the tube voltage to 40 kV,and the same procedure as above is followed to estimate the maximum peakvalue H2. Similarly, an estimation is made on the maximum peak value H3when the tube voltage is kV and the maximum peak value H4 when the tubevoltage is kV. It takes, for example, eight hours of measurement time toobtain the four maximum peak values H.

This makes it possible for all detecting elements 6 to exactly outputthe photon energy values of the X-ray photons having energy values of atleast 20 keV, 40 keV, 60 keV, and keV. Therefore, the X-ray device 2 canaccurately determine which energy bin the energy value of the photonsdetected by the detecting element 6 belongs to, and accurately grasp thepeak value h corresponding to the energy value that forms the boundaryof that energy bin for each detecting element 6.

As illustrated in FIG. 7 , the detector 4 is designed so that the energyvalue of the photons incident on the detecting element 6 and the peakvalue of the electrical pulse signal outputted from the detectingelement 6 usually ideally have a proportional relationship. In FIG. 7 ,the ideal characteristic of the detecting element 6 is indicated by adashed-dotted line for explanation. However, in reality, as indicated bythe solid line, the characteristic may differ from the proportionalrelationship, and the characteristic may differ for each detectingelement 6. In addition, there are cases where the proportionalrelationship is not achieved at the design stage, and the actualcharacteristic may deviate from the design due to manufacturingconvenience or other causes.

Ideally, the characteristic indicated by the solid line is acquired foreach detecting element 6 and calibrated so that an accurate energy valuecan be acquired from the detected peak value. In this case, by changingthe tube voltage by 1 kV stepwise, for example, and repeating theestimation of the maximum peak value H and the acquisition of thecalibration value d, it is possible to acquire the characteristicindicated by the solid line in FIG. 7 .

On the other hand, when performing calibration by estimating the maximumpeak values H1 to H4 limited to the energy values forming the boundariesof energy bins, it is sufficient to acquire the calibration values donly in the vicinity of each of H1 to H4, which significantly reducesthe time required for calibration. Further, the detecting element 6 canbe accurately calibrated at an energy value that is the boundary ofenergy bins. The X-ray device 2 can accurately discriminate which energybin the energy value of the photons detected by the detecting element 6belongs to, and it is possible to accurately grasp the peak valuecorresponding to the energy value that forms the boundary of that energybin for each detecting element 6. Since the measurement accuracy of theX-ray device 2 can be improved, the X-ray device 2 can output highlyaccurate images.

The calibration device 1 can accurately calibrate the detecting element6 at any energy value. Therefore, even in the detector 4 having adifferent energy value being the boundary of energy bins from the above,calibration can be performed with high accuracy. A simple control ofchanging the tube voltage applied to the X-ray tube 3 makes it possibleto change the energy value to be calibrated. The calibration device 1can accurately calibrate various detectors 4.

The calibration method will be described more specifically by taking thedetector 4 illustrated in FIG. 8 as an example. The detector 4 of thisembodiment includes detecting elements 6 each composed of, for example,CdZnTe (Cadmium Zinc Telluride) semiconductor and a photon counting typeASIC connected to the detecting elements 6. The photon counting typeASIC includes a charge amplifier 11 that receives and amplifies chargefrom the detecting elements 6, a waveform shaper 12 that shapes thewaveforms of signals received from the charge amplifier 11, adiscriminator 13 that discriminates the signals received from thewaveform shaper 12, a counter 14 that counts the signals outputted fromthe discriminator 13, and a DA converter 15 that sends the signals, sentfrom the control mechanism 5, to the discriminator 13. In FIG. 8 , thesignal lines are indicated by dashed-dotted lines for explanation. Inaddition, the traveling direction of signals is indicated by arrows.

When X-rays are incident on the detecting element 6, a chargeproportional to the energy of the X-rays is generated. This charge issent to the charge amplifier 11. The charge amplifier 11 outputs awaveform with a peak proportional to the amount of charge. The waveformshaper 12 smoothly shapes the waveform received from the chargeamplifier 11. If the waveform has fine vibrations due to noise or otherfactors, the accuracy of subsequent discriminations by the discriminator13 will be reduced.

Four discriminators 13 are connected to the waveform shaper 12. Thediscriminator 13 is configured such that it has a reference voltage setin advance and extracts only waveforms having a voltage higher than thereference voltage. The reference voltage is set, for example, to avoltage corresponding to the energy value that is the boundary of energybins. In reality, the reference voltage is set as the channel numbercorresponding to an energy value.

For example, if the channel number has a configuration of 0 to 127 (7bits) and the upper limit of the energy value processed by thediscriminator 13 is 80 keV, the range is 0.625 keV per channel. Forexample, when the energy values serving as the boundaries of the energybins are set to 20, 40, 60, and 80 keV in the four discriminators 13,the channel numbers set to the discriminator 13 are 32, 64, 96, and 127,respectively. Ideally, the channel number is proportional to the energyvalue. Also, ideally, the channel number has a proportional relationshipwith the peak value h.

For example, when an X-ray photon having an energy value of 30 keV isincident on the detecting element 6, the discriminator 13 recognizes itas a photon with channel number 48. In the discriminator 13 whosechannel number is set to 32, a digital signal corresponding to channelnumber 48 is sent to the counter 14. In the discriminator 13 whosechannel number is set to 64, no signal is sent to the counter 14 becauseit is below the reference voltage.

The counter 14 is configured to count the digital signals outputted fromthe discriminator 13. One counter 14 is connected to one discriminator13. In this embodiment, the detector 4 has four counters 14. The counter14 is configured to count the photon count C for each channel number andsend the result to the control mechanism 5.

The charge amplifier 11, the waveform shaper 12, the four discriminators13, and the four counters 14 are arranged for each detecting element 6.That is, the mechanisms other than the DA converter 15 in the photoncounting type ASIC illustrated in FIG. 8 are provided for each of theplurality of detecting elements 6.

The DA converter 15 is configured to convert the signals sent from thecontrol mechanism 5 and set a reference voltage for each discriminator13. The DA converter 15 is configured to collectively set referencevoltages for the plurality of discriminators 13 included in the photoncounting type ASIC.

The control mechanism 5 may have the fine adjustment mechanism 16. Thefine adjustment mechanism 16 is configured to finely adjust thereference voltage by sending a signal to the discriminator 13. Inreality, the reference voltage is finely adjusted as a trim number.Adjustment of the reference voltage by the fine adjustment mechanism 16can be performed with different values for each discriminator 13. Thefine adjustment mechanism 16 can be adjusted with different trim numbersfor the plurality of detecting elements 6.

The calibration device 1 calibrates the energy value by adjusting thereference voltage of the discriminator 13. If the upper limit of theenergy value measurable by the detector 4 is, for example, 80 keV,ideally, channel number 0 corresponds to an energy value of 0 keV andchannel number 127 corresponds to an energy value of 80 keV.

If X-rays are measured at a tube voltage of 40 kV at this time,measurement results similar to the graph illustrated in FIG. 5 areobtained. Ideally, the photon count C for channel number 64 is 1 ormore, and the photon count C for channel number 65 or more is 0. Thatis, it can be estimated that channel number 64 is the closest integervalue to the maximum peak value H. It is assumed that the energy valuecorresponding to channel number 64 is 40 keV.

Next, the calibration device 1 estimates the maximum peak value H foreach detecting element 6. In actual measurement, some of the detectingelements 6 output the maximum peak value H as channel numbers 62 and 66.FIG. 9 illustrates the relationship between the channel number estimatedas the maximum peak value H and the number of detecting elements 6therefor. For X-rays having an energy value of 40 keV, variation occursin the channel number outputted from each detecting element 6. If themedian value of the graph is channel number 66 as illustrated in FIG. 9, the channel number is adjusted via the control mechanism 5. Channelnumber 66 is set as the boundary of energy bins corresponding to anenergy value of 40 keV.

That is, the calibration device 1 adjusts the reference voltage of thediscriminator 13 via the control mechanism 5. The channel numbercorresponding to the energy value of 40 keV is changed from 64 to 66 inthe discriminator 13.

The detector 4 before calibration was in a state where when the channelnumber outputted from discriminator 13 was 64, the corresponding energyvalue was assumed to be 40 keV. It can be seen from the measurement bythe calibration device 1 that the energy value was actually below 40 keVwhen the channel number was 64. The channel number 66, as the obtainedcalibration value d, is sent from the calibration device 1 to thecontrol mechanism 5. For the detector 4 after calibration, when thechannel number outputted from the discriminator 13 is 66, thecorresponding energy value is 40 keV. This detector 4 can accuratelymeasure the energy value of 40 keV.

The calibration device 1 may be configured to acquire data correspondingto the graph illustrated in FIG. 9 and extract the median value. In thiscase, the calibration device 1 works on the control mechanism 5 to resetthe channel number. Specifically, the reference voltage of thediscriminator 13 is changed from the control mechanism 5 via the DAconverter 15, based on the calibration value d sent from the calibrationdevice 1.

For X-rays having an energy value of 40 keV, a large number of detectingelements 6 are in a state of outputting channel number 66. When channelnumber 66 is outputted, the measured X-ray photons will have an energyvalue of approximately 40 keV. One reference voltage corresponding tochannel number 66 is set for all of the plurality of detecting elements6.

If the control mechanism 5 includes the fine adjustment mechanism 16,the calibration accuracy can be further improved. As illustrated in FIG.9 , some of the detecting elements 6 output channel number 64 other thanchannel number 66, which gives the maximum peak value H. The channelnumber varies between 64 and 68. In other words, some of the detectingelement 6 output the X-ray photons of 40 keV as 38.75 to 41.25 keV.

The fine adjustment mechanism 16 fine tunes the reference voltage as atrim number for the energy value. For example, a case where the trimnumber has a configuration of 0 to 15 (4 bits) will be described. Asillustrated in FIG. 9 , when there is a ±2 variation range with respectto the median value channel number 66, the range corresponding to fourchannel numbers is first set as the trim range. The range per trimnumber corresponds to a range of 0.25 channel numbers.

The maximum peak value H estimated for each detecting element 6 by thecalibration device 1 is calculated as a channel number. The channelnumber is a digital value and a numerical value with only an integerpart, but the channel number estimated as the maximum peak value H has anumerical value including a decimal part.

In a certain detecting element 6, where the channel number set from thecontrol mechanism 5 to the DA converter 15 is 66, and the initialsetting value of the trim number is 7, and the maximum peak value H isestimated to be at channel number 66.5, the trim number of thisdetecting element 6 is set to +2. This trim number +2 is sent from thecalibration device 1 to the control mechanism 5 as the obtainedcalibration value d. As a result, the reference voltage in thediscriminator 13 increases by the same amount as the channel number of0.5. That is, in this detecting element 6, the channel number 66.5coincides with the energy value of 40 keV. This detecting element 6counts the photon count C using the channel number 66.5 as the boundaryof energy bins of 40 keV. A trim number is determined for each of thedetecting elements 6 in the same manner as described above.

The calibration device 1 may be configured to determine the trim rangebased on variations in the maximum peak value H for each detectingelement 6. Further, the calibration device 1 may be configured todetermine the trim number based on the estimated maximum peak value H.Specifically, the fine adjustment mechanism 16 of the control mechanism5 finely adjusts the reference voltage of the discriminator 13 based onthe calibration value d sent from the calibration device 1.

The channel number corresponding to the actual peak value H used fordetermination by the discriminator 13 is a value very close to thechannel number (integer) set in the DA converter 15 from the controlmechanism 5. If the boundary of energy bins is set at 40 keV, thenphotons with energy values 40 keV or less are counted as being includedin the bin below the boundary. At this time, the channel numbercorresponding to the actual peak value H used for determination is avalue very close to 66. Photons with energy values greater than 40 keV,on the other hand, are counted as being included the bin above theboundary. At this time, the channel number corresponding to the actualpeak value H used for determination is a value very close to 66. With anenergy value of 40 keV as a boundary, photon energy values arediscriminated with high accuracy.

The calibration device 1 may have a configuration such that aftercalibration, the calibration device 1 performs re-measurement andevaluates the calibration results. After calibration by the fineadjustment mechanism 16, all the detecting elements 6 fall within thechannel number range of 66±0.25, for example. At this time, the resultillustrated by the dashed line in FIG. 9 is obtained. All detectingelements 6 can detect X-ray photons having an energy value of 40 keVwith an accuracy of 40 keV ±0.16 keV.

In this embodiment, the calibration unit 10 calculates the channelnumber and trim number set by the calibration device 1 as thecalibration value d. By sending this calibration value d from thecalibration device 1 to the control mechanism 5, it is possible toaccurately perform energy value calibration in the X-ray device 2.

If the correspondence relationship between the trim number of the fineadjustment mechanism 16 and the channel number is out of proportion, itis conceivable that the energy value after calibration will not convergeas expected. Even in such a case, the accuracy of calibration can beimproved by repeating the setting and measurement (calibration) of thechannel number and trim number. The configuration may be such that ifthere is a detecting element 6 whose energy value after calibrationcannot converge, it is treated as a defective pixel. The calibrationdevice 1 sends defective pixel information such as positionalinformation for identifying defective pixels to the control mechanism 5as a calibration value d. The configuration may be such that in theX-ray device 2, the control mechanism 5 eliminates the signal obtainedfrom the detecting element 6 determined as a defective pixel based onthe defective pixel information. The control mechanism 5 becomes a stateof not using signals obtained from the detecting element 6 determined asa defective pixel. In an image analyzer using the detector 4, by notusing the data of defective pixels during data analysis, it is possibleto prevent the inclusion of relatively large error data in energyinformation and improve the analysis accuracy.

The calibration method by the calibration device 1 is not limited to theabove. If the detector 4 has a different configuration, for example,such as in a case of a photon counting type ASIC, the calibration valued obtained by the calibration device 1 and the calibration method willchange. It suffices that the calibration device 1 uses the maximum peakvalue H to accurately calibrate the relationship between the signaloutputted from each detecting element 6 and the corresponding energyvalue.

As illustrated in FIG. 10 , the configuration may be such that thecalibration device 1 includes a filter mechanism 17 arranged between theX-ray tube 3 and the detector 4. The filter mechanism 17 has aconfiguration in which a plurality of filters made of differentmaterials are arranged in a switchable manner. The filter is, forexample, a filter made of aluminum with a thickness of 4.0 mm, or afilter made of a combination of aluminum with a thickness of 2.0 mm andcopper with a thickness of 0.3 mm. In this embodiment, a plurality offilters made of different materials are arranged and fixed in adirection orthogonal to the X-ray irradiation direction. By moving inthe direction of arranging the filters, the filter mechanism 17 canswitch the filters through which X-rays pass. In FIG. 10 , the directionof moving the filter is indicated by an arrow for explanation.

The filter mechanism 17 may be connected via a signal line to a filtercontrol unit 18 that automatically controls switching of filters. InFIG. 10 , the signal lines are indicated by dashed-dotted lines forexplanation. The filter control unit 18 acquires the tube voltage fromthe X-ray tube control unit 7 and automatically switches to a presetfilter according to the tube voltage. When the tube voltage of the X-raytube 3 is changed, the filters of the filter mechanism 17 are switched.Filter switching may be done manually. In this case, the calibrationdevice 1 is configured without the filter control unit 18.

The filter of the filter mechanism 17 can change the quality of X-rayspassing therethrough. The filter is selected from materials thatincrease the photon count in the extraction region S, which is useful inestimating the maximum peak value H, and decrease the photon count inthe portion not included in the extraction region S. Therefore, theoptimum filter material differs for each tube voltage.

For example, when the tube voltage is 20 kV, a 0.7 mm aluminum filter isused; when it is 40 kV, a filter made of a combination of 2.0 mmaluminum and 0.3 mm copper is used; and when it is 60 kV, a filter madeof a combination of 2.0 mm aluminum and 1.4 mm copper is used. When theX-ray device 2 is calibrated, the filter of the filter mechanism 17 isswitched according to the tube voltage when changing the tube voltage.

Since the photon count in the extraction region S increases asillustrated in FIG. 11 , the accuracy in estimating the maximum peakvalue H can be improved. Moreover, since the photon count in the energyrange required for estimating the maximum peak value H is relativelyincreased, it is possible to shorten the time required for calibration.

As the dose of X-rays emitted from the X-ray tube 3 is reduced, the timeintervals between the electrical pulse signals detected by the detectingelement 6 increase, making it easier to suppress pile-up. On the otherhand, since the X-ray photon count emitted from the X-ray tube 3decreases, the time required for calibration increases when trying toobtain a certain photon count. By utilizing the filter mechanism 17, itis possible to increase the count of photons required for estimating themaximum peak value H without increasing the total dose of incidentradiation on the detector 4, even if the dose from the X-ray tube 3 isincreased. This is advantageous for shortening the time required forcalibrating the X-ray device 2.

If the calibration device 1 includes the filter control unit 18, thecalibration of the X-ray device 2, which takes several hours, can beautomatically performed. This is advantageous for efficientlycalibrating the detector 4 before shipment in a manufacturing factory ofthe detector 4 or the like.

When the calibration device 1 is incorporated in the X-ray device 2, itbecomes possible to automatically calibrate the X-ray device 2 at nightwhen food production is stopped in a food factory or the like.

EXPLANATION OF REFERENCE NUMERALS

-   -   1 calibration device    -   2 X-ray device    -   3 X-ray tube    -   4 detector    -   5 control mechanism    -   6 detecting element    -   7 X-ray tube control unit    -   8 acquisition unit    -   9 calculation unit    -   10 calibration unit    -   11 charge amplifier    -   12 waveform shaper    -   13 discriminator    -   14 counter    -   15 DA converter    -   16 fine adjustment mechanism    -   17 filter mechanism    -   18 filter control unit    -   p pulse interval    -   w pulse width    -   H maximum peak value    -   h peak value    -   d calibration value    -   S extraction region    -   C photon count

1. A calibration device for a detector including a plurality ofdetecting elements each of which generates an electrical pulse signalwith a peak value corresponding to an energy value of incident X-rayphotons and counts a photon count for each peak value, the calibrationdevice comprising: an X-ray tube control unit that controls a tubevoltage of an X-ray tube for irradiating the detector with X-rays; anacquisition unit that acquires the photon count for each peak value fromeach of the detecting elements and acquires the tube voltage of theX-ray tube; a calculation unit that estimates a maximum peak valuewithin a range in which X-ray photons are detected from values obtainedby the acquisition unit and at which the peak value is maximum; and acalibration unit that calculates, for each of the detecting elements, acalibration value that associates the tube voltage acquired by theacquisition unit with the maximum peak value, wherein the X-ray tubecontrol unit is configured to change the tube voltage in a statecorresponding to an energy value serving as a boundary of a plurality ofpredetermined ranges of energy values, and the calibration unit isconfigured to calculate a calibration value for each tube voltage. 2.(canceled)
 3. The calibration device according to claim 1, wherein thecalculation unit is configured to determine an approximate straight linebased on a relationship between the photon count and the peak value,within a predetermined range that is smaller than the peak valuecorresponding to the tube voltage acquired by the acquisition unit, anddesignates a value of the peak value when the photon count becomes zeroon the approximate straight line as the maximum peak value.
 4. Thecalibration device according to claim 1, wherein the calculation unit isconfigured to determine an approximate straight line based on arelationship between a square root of the photon count and the peakvalue, within a predetermined range that is smaller than the peak valuecorresponding to the tube voltage acquired by the acquisition unit, anddesignates a value of the peak value when the photon count becomes zeroon the approximate straight line as the maximum peak value.
 5. Acalibration method for a detector including a plurality of detectingelements each of which generates an electrical pulse signal with a peakvalue corresponding to an energy value of incident X-ray photons andcounts a photon count for each peak value, the calibration methodcomprising: applying a predetermined tube voltage to an X-ray tube forirradiating the detector with X-rays; acquiring the photon count foreach peak value from each of the detecting elements; estimating amaximum peak value within a range in which X-ray photons are detectedand at which the peak value is maximum; calculating, for each of thedetecting elements, a calibration value that associates the tube voltagewith the maximum peak value; applying a tube voltage corresponding toone of energy values serving as a boundary of a plurality ofpredetermined ranges of the energy values to the X-ray tube to acquirethe calibration value for each of the detecting elements; and applying atube voltage corresponding to another of the energy values serving as aboundary of the plurality of predetermined ranges of the energy valuesto the X-ray tube to acquire the calibration value for each of thedetecting elements.
 6. (canceled)
 7. The calibration method according toclaim 5, further comprising: determining an approximate straight linebased on a relationship between the photon count and the peak value,within a predetermined range that is smaller than the peak valuecorresponding to the tube voltage; and designating a value of the peakvalue when the photon count becomes zero on the approximate straightline as the maximum peak value.
 8. The calibration method according toclaim 5, further comprising: determining an approximate straight linebased on a relationship between a square root of the photon count andthe peak value, within a predetermined range that is smaller than thepeak value corresponding to the tube voltage; and designating a value ofthe peak value when the photon count becomes zero on the approximatestraight line as the maximum peak value.